With the densification of integrated semiconductor circuits in recent years, printed wires in the circuits have been extremely fined to widths of from 1 to 3 .mu.m and the lithographic technique of forming images of such wires on wafers are being changed from the projection system to the stepper system.
When dust is present on a photomask reticle in the lithographic process, images of dust particles will also be formed, resulting in short circuits and defects in the produced circuit, and this will lower the yield of the LSI. Specially in the stepper system, several images on a reticle are successively projected on a reduced scale to a wafer. Hence one dust particle on a reticle may make defective all the resulting LSI's. Thus it is getting very important to reduce the dust to zero.
Thereupon, a method for preventing dust adhesion onto photomask reticles (hereinafter abbreviated occasionally as "mask") has been proposed (U.S. Pat. No. 4,131,363). The method comprises arrangement of a transparent film or films over one or both sides of the mask with a space left between the film and the mask surface. According to this method, since dust adheres only to the transparent film, defects due to dust can be prevented by focussing projecting rays on the image prepared on the mask and bringing the dust adhering onto the film out of focus, so that the image of the dust is not formed on the wafer.
The dust cover, applied to the mask, is set in an exposer. The film, which is the principal part of the dust cover, is disposed in the optical path of exposure light.
Consequently, the film needs to transmit light without causing the distortion or disorder of image, have a uniform thickness, and be free of foreign matter, fault, and internal strain.
The transparency of the film to the exposure light is also important. That is, when the transparency is low, the exposure needs to be the longer and therefore the throughput will be the less. In the production of LSI, it is very important to raise the throughput since LSI's in many cases are produced in extremely large volumes. Specially in the stepper system, the exposure in some cases is repeated hundreds of times per one wafer and the improved transparency of the dust cover film hence contributes greatly to the rise in the throughput.
For the film of the dust cover, nitrocellulose is in use. Although nitrocellulose is used because of the high film strength thereof and the capability thereof to give very uniform films, the light transmittance of these films is about 92% at wavelengths of from 350 to 450 nm which are of the exposure light used today. Thus nitrocellulose films as such are insufficient in light transmittance for use as the film of the dust cover. In consequence, a method has been proposed, in which the interference of two light waves reflected from the front and rear surfaces of the film is utilized, that is, the film thickness is chosen so that these reflected waves will cancel each other (U.S. Pat. No. 4,378,953). According to this method, the light transmittance rises to 99% while the film thickness becomes 0.865 .mu.m, being extremely thin. Hence this method has the drawback of requiring the exercise of sufficient care in handling such a dust cover. On the other hand, the increase in the film thickness will extremely narrow the tolerance of the thickness in order to achieve a light transmittance of 98% or more, thus making the film production very difficult. In addition, if the film thickness is increased to 6 .mu.m or more, the light transmittance thereof will be lowered on account of minute dimensional roughness of the film surface.
Another method is to prevent the reflection with an inorganic compound layer formed by vapor deposition or the like on both sides of a nitrocellulose film. This method is an application of the technique used for optical lens and eyeglasses. According to this method, the reflection of light of any possible wavelength as desired can be prevented by choosing the thickness of the anti-reflection layer and a light transmittance of 98% or more can be attained for a relatively thick film.
In order to form such films, however, an expensive apparatus such as a vacuum deposition arrangement is necessary and additionally the vapor deposition needs to be carried out at a low temperature so as not to impair the cellulose film. In the low temperature vapor deposition, the inorganic compound to coat is not densely deposited and the refractive index of the deposit layer varies with the temperature condition. Hence the deposit layer may not exhibit the intended anti-reflection function. Moreover, the deposited material, depending on the nature thereof, may be oxidized with air, resulting in a change in the refractive index. Accordingly, the function of the resulting dust covers is not definite.
On the other hand, as LSI's are more densified, there is a growing demand for printed wire widths of submicron order. To meet such a demand, rays of wavelengths from 350 to 450 nm used today for the exposure do not have enough resolving power and rays of shorter wavelengths, i.e. ultraviolet rays of wavelengths from 240 to 290 nm need to be used. However, nitrocellulose films show rapid decrease in the light transmittance with decrease in wavelength from 300 nm and are degraded by far ultraviolet rays to such an extent that it can be no longer used. Therefore a novel dust cover is looked for.